The present invention relates to a punch press and more particularly to a variable stroke punch press in which the eccentricity of the drive, and therefore the length of the stroke, is adjusted by rotation of the crankshaft relative to the eccentric sleeve.
The stroke of a crankshaft, and therefore the displacement of the slide member, is defined as the distance the crankshaft moves between its top dead center and bottom dead center positions. This distance determines the maximum depth of drawing when the press is used to implement a drawing process, the maximum height available for a container edge turning process, and the maximum length of the article produced by an ironing or a backward extrusion process, for example.
The stroke length of the press is determined by the application, so a variety of applications require different stroke lengths. Many presses are designed having only one stroke length and are therefore not suited for different applications without substantially repositioning the tooling. In order to perform different applications utilizing only one press, it is known to provide a mechanism to change the stroke length by varying the eccentricity. As shown in U.S. Pat. No. 2,454,881 issued Nov. 30, 1948 to Michelman, a change in stroke length is accomplished by rotating the eccentric relative to the stopped crankshaft. The disadvantage is that the eccentric is buried within the crown of the press, surrounded by the housing and obscured by connection arms, hoses, belts, etc., whereby it is extremely difficult to reach and move the eccentric.
As shown in U.S. Pat. No. 4,785,732 issued Nov. 22, 1988, to Czapka et al., a change in stroke length is accomplished by fixing the eccentric with a bolt supported in the machine frame and rotating the crankshaft relative to the eccentric. Such mechanisms to fix the eccentric to the press frame are often complicated and unwieldy.
It is therefore desirable to provide a variable stroke punch press which overcomes the above disadvantages such that it would be simple to change the stroke length as well as easy to construct and maintain.